Alzheimer's disease is one of the most common causes of dementia in man. It is a chronic and fatal disease associated with neural cell degeneration in the brain of the affected individual, characterized by the presence of amyloid plaques consisting of extracellular deposits of amyloid β-peptide (Aβ-peptide). The neural cell atrophy caused by Aβ aggregation results in deficiency of acetylcholine and other signaling substances. It is known that Aβ-peptide, having 40-42 amino acid residues, is produced by processing of the amyloid precursor protein (APP), which is a membrane protein normally expressed by the neurons of the central nervous system, but the reasons for this processing are incompletely understood. The released Aβ peptide contains a part of the transmembrane region of APP (Aβ residues 29-40/42) and includes a discordant helix, i.e. a helix composed of amino acids with a high propensity to form β-strands. Aβ is prone to misfold and aggregate when removed from its stabilising membrane environment.
Current therapeutic approaches for treatment of Alzheimer's disease are mainly directed to treating the symptoms and include cholinergic replacement therapy, e.g. inhibition of acetylcholinesterase, small inhibitors that interact with soluble Aβ oligomers, and so-called β-sheet breakers that prevent elongation of already formed β-sheet structures.
Another suggested strategy to prevent aggregation has been to utilize molecules that are functionally defined as chaperones. Chaperones play an important role by aiding the correct folding of proteins in the complex intracellular milieu. A number of molecular chaperones, such as heat-shock proteins (Hsp), are known to be important in the folding process and have been extensively studied. Some of these chaperones are apparently able to interact with and have an impact on the amyloid fibril formation of certain polypeptides. Aggregation of Aβ1-42 is inhibited by Hsp90 or the combination Hsp70/Hsp40 (C G Evans et al, J Biol Chem 281: 33182-33191, 2006). Furthermore, the extracellular chaperone clusterin (apolipoprotein J) has been shown to inhibit fibril formation of a number of polypeptides including Aβ (E Matsubara et al, Biochem J 316 (Pt 2): 671-679, 1996) and a fragment of the prion protein (S McHattie and N Edington, Biochem Biophys Res Commun 259: 336-340, 1999). The role of the structurally diverse chaperones in prevention of amyloid diseases is not established and some reports even indicate that protein chaperones promote amyloid fibril formation, see e.g. S K DebBurman et al. Proc Nat Acad Sci USA 94: 13938-13943, 1997. In addition to molecular chaperones, the effects of chemical and pharmacological chaperones have been studied in the context of misfolding diseases. No effective therapy using chaperones or other means has so far been found for any amyloid disease.
Monoclonal antibodies against Aβ peptide prevent aggregation into neurotoxic fibrils and dissolve already formed amyloid. However, antibody therapy is very costly and associated with side-effects of varying seriousness. Vaccination with β-amyloid in transgenic mice models of Alzheimer's disease has shown a significant reduction in the number of amyloid plaques and overall amyloid burden and even some improvement in cognitive performance.